L12 precipitation-strengthened multi-principal-element alloys (MPEAs) exhibit outstanding quasi-static tensile properties at both ambient (298 K) and cryogenic (77 K) temperatures, highlighting their potential for engineering applications. However, their impact behaviors remain poorly understood. In this work, the tensile and Charpy impact behaviors of (Fe58.98Ni31.7Al6Ti3Zr0.1C0.2B0.02)100-xCrx (x = 0, 4, 8, and 13 at.%) at 298 K and 77 K are systematically investigated. The Cr8 alloy achieves the optimal balance of strength, ductility and Charpy impact toughness at both 298 K and 77 K. Notably, the Cr8 alloy delivers exceptional mechanical performance at 77 K, with a yield strength of 927 ± 13 MPa, ultimate tensile strength of 1552 ± 9 MPa, uniform elongation of 48.8 ± 3.0%, and Charpy impact toughness of 87.9 ± 3.4 J/cm2. When the Cr content exceeds 8 at.%, the yield strength, ultimate tensile strength, and room-temperature Charpy impact energy of the alloys exhibit a slight decreasing trend. This composition-dependent performance is attributable to Cr-induced modulation of the dominant deformation mechanism. The Cr0 alloy exhibits FCC to BCC transformation; the Cr4 alloy shows co-dominance of deformation twinning and dislocation slip; while the Cr8 and Cr13 alloys are governed by stacking faults and dislocation slip. These results demonstrate that Cr alloying is an effective strategy to tailor the strength, ductility, and impact toughness of L12-precipitate-strengthened Fe-based MPEAs at 298 K and 77 K.
To improve the strength–ductility synergy of CoCrFeNiCu alloys, a composition design strategy coupling molecular dynamics (MD) with machine learning (ML) is proposed. A dual-objective XGBoost model trained on 600 MD tensile cases (298–873 K) reached cross-validated R2 of 0.957 for strength and 0.821 for ductility, and three rounds of active learning (prediction deviation ≤ 0.41%) identified three optimized compositions on the Pareto front. The two high-ductility alloys share a common signature: Co enrichment to 31.9–35.2 at.% and Cu depletion to 5.7–12.0 at.%. Relative to the equiatomic baseline they show 27–28% higher ductility and a 32% higher strength–ductility product over 298–873 K. The gain arises from a deformation-induced FCC-to-BCC shear transformation that begins within the uniform deformation stage, at onset strains of 0.139–0.140 against 0.172–0.179 for the baseline, and reaches peak BCC fractions of 38–54% against 9–13%. The transformation restores work hardening after the initial stress drop, which the baseline alloys do not exhibit. Nucleation proceeds by stress-induced homogeneous shear without a dislocation precursor, and the transformation reverts almost completely on unloading via Shockley partials. The bimodal ductility distribution predicted by ML corresponds strictly to this mechanistic dichotomy, showing that the model locates metastable composition domains without prior mechanistic input.
Hot extrusion is an effective route to tailor the microstructure and mechanical performance of TiAl intermetallic alloys. In the present work, a Ti-45Al-8Nb-0.2W-0.1B alloy was multi-pass extruded at five temperatures spanning the (α + γ) two-phase region to the α single-phase region. The influences of extrusion temperature on microstructural evolution, room-temperature and high-temperature tensile properties, as well as fracture and deformation mechanisms were systematically investigated via SEM, EBSD and TEM characterizations. With increasing extrusion temperature, the volume fraction and average diameter of lamellar colonies continuously increase, and the microstructure gradually evolves from near-gamma to fully lamellar. The duplex microstructure exhibits a favorable combination of strength and ductility at both room and elevated temperatures. Fracture mode shifts from inter-granular fracture for near-gamma microstructure to inter-lamellar/trans-lamellar fracture for near/fully lamellar microstructure. Dislocation slip and twinning are the dominant deformation mechanisms at both room temperature and elevated temperature. However, at elevated temperature, a limited amount of dynamic recrystallization of γ grains and a stress-induced α2 → ωo phase transformation are also observed, which locally contribute to stress relaxation and improved high-temperature plasticity. This work provides experimental guidance for microstructure optimization of high-Nb TiAl alloys by regulating extrusion temperature.
Two NiCoCr-based medium-entropy alloys with different carbon and nitrogen interstitial contents were fabricated by laser powder bed fusion (LPBF). Alloy-specific processing parameters were optimized through a combined assessment of porosity, relative density, surface morphology, and surface roughness, and the selected samples were subsequently annealed over a range of temperatures. X-ray diffraction confirmed that both alloys retained a single-phase FCC structure under the investigated conditions, while differences in peak positions indicated distinct lattice responses. Electron backscatter diffraction (EBSD) revealed contrasting microstructural evolution during annealing. The lower-interstitial alloy underwent heterogeneous recrystallization accompanied by partial strain relaxation, grain refinement, and extensive annealing twinning. In comparison, the higher-interstitial alloy showed more limited recrystallization and pronounced strain localization, reflected in increased kernel average misorientation and apparent geometrically necessary dislocation density. The hardness and microstructural results indicate distinct annealing and strengthening responses in the two alloys. The higher interstitial content of the second alloy is associated with more spatially localized lattice curvature and more limited recrystallization. This study links the contrasting annealing responses of LPBF-processed NiCoCr-based alloys to their EBSD-resolved recrystallization, twinning, and strain-localization behavior, providing new insight into the relationship between composition, microstructural evolution, and mechanical response.
